Forming articles from sheet material

ABSTRACT

In a trim in place thermoforming process for shaping articles from sheet material wherein a blade penetrates the sheet around the periphery of each article to facilitate later removal, method and apparatus improvements wherein limited portions of the leading surface of the blade are introduced into the sheet to a lesser extent than that of the remainder of the leading surface to form frangible tabs in the sheet adjacent side portions of the article which extend in the direction of sheet advancement, thereby reducing the criticality of the sheet thickness along the line of penetration for retaining the articles attached to the sheet. To form the tabs, the leading surface of the blade has indentations therein which are preferably oriented at an acute angle to the adjoining surface of the article being formed, as opposed to the adjoining surface of the portion of the sheet remaining after the article is removed.

United States Patent 1191 Layman FORMING ARTICLES FROM SHEET MATERIAL[75] inventor: Lee R. Layman, Windsor Locks,

Conn.

[73] Assignee: Monsanto Company, St. Louis, Mo. 22 Filed: April 2, 1970[21] Appl, No 25,186

[52] US. Cl ..425/291, 425/385 51 1111.01 ..B29c 17/00 [58] Field ofSearch....l8/l9 R, 19 BM, 19 BE, 19 F, 18/35; 264/89, 322; 425/385, 109,291

Primary ExaminerRobert L. Spicer, Jr. Attorney-James C. Logomasini,Michael J. Murphy and Neal E. Willis 1451 Apr. 17, 1973 ABSTRACT In atrim in place thermoforming process for shaping articles from sheetmaterial wherein a blade penetrates the sheet around the periphery ofeach article to facilitate later removal, method and apparatusimprovements wherein limited portions of the leading surface of theblade are introduced into the sheet to a lesser extent than that of theremainder of the leading surface to form frangible tabs in the sheetadjacent side portions of the article which extend in the direction ofsheet advancement, thereby reducing the criticality of the sheetthickness along the line of penetration for retaining the articlesattached to the sheet. To form the tabs, the leading surface of theblade has indentations therein which are preferably oriented at an acuteangle to the adjoining surface of the article being formed, as opposedto the adjoining surface of the portion of the sheet remaining after thearticle is removed.

2 Claims, 4 Drawing Figures PATENTEB APR 1 71975 FIG. 4

INVENTOR.

LEE R. LAYMAN 1 FORMING ARTICLES FROM SHEET MATERIAL BACKGROUND OF THEINVENTION This invention relates to the forming of articles fromthermoplastic sheet material, and in particular to improvements inretaining articles attached to the sheet after trimming and prior toseparation in a trim in place thermoforming operation.

Several types of forming are known for simultaneously shaping pluralarticles in recurring cycles from portions of a generally continuousplastic sheet. One technique is by pneumatic pressure differentialforcing ,of portions of a heated area of the sheet against the surfacesof mold cavities to form depressions therein using sealing blades tohold the pressure in each mold during forming, followed by a creasing orscoring of the sheet around the formed articles at the seal while stillin the press by means of the blades advancing further into the sheetwhich is trapped against an opposing firm surface. This creasingcorresponds to what is known as a trimming step, in that the bladepenetrates almost but not entirely through the full thickness of thesheet, the unpenetrated portion of the thickness permitting transfer ofthe articles out of the mold and away from the forming station whilestill held in the sheet. Thereafter, the sheet with the articlesretained therein is passed across a stripping surface such as a roll forthe purpose of shearing the articles out of the sheet along thepreviously formed crease or score lines, whereupon the scrap sheet, orweb as it is known in the art, is accumulated or immediately reprocessedas desired. U.S. Pat. No. RE 26,413 illustrates a machine operating onthese principles. This type of forming is referred' to as a type of trimin place thermoforming, as opposed to post trim thermoforming wherein aseparate trimming press is required downstream of the forming station toseparate the articles from the sheet, or the trim in place techniquewherein the articles are entirely severed from the sheet while in theforming press and then must be individually removed therefrom. From thestandpoint of speed and economy, trim in place thermoforming of thefirst mentioned variety is highly desirable since the portion of thesheet or web between the mold cavities is used to transport thearticles, for example containers, out of the forming press while thearticles are still attached therein.

As can be expected, thesheet contacting surfaces of the sealing bladesacting against the opposing platen surface will become blunt with wearafter numerous cycles of the equipment. Also, over a period of time, oneor more blades in a press may need replacing whereas the remainder mightnot, with the result being that the leading surfaces of the blades maynot all be aligned in a common plane after reassembly. Either of theseconditions will produce variable levels of sheet penetration during thesecond trimming stroke of the sealing blades. If substantial portions ofthe sheet have not been sufficiently penetrated around each article,when it is directed across the stripping roll the articles instead ofcleanly snapping out of the sheet do so only partially and tend toundesirably follow the web, or else discharge erratically from the sheetin a misalinged manner.

On the other hand, if penetration is excessive, rather than strippingout of the mold cavity as is desired when the platen on which the moldsare mounted moves away after the forming cycle, the articles tend tohaphazardly and prematurely break away from the sheet and hang up in themolds. Also, as the rate of sheet advance is increased to reduce cycletime and increase capacity, the rate of acceleration and deceleration ofthe sheet during indexing between the forming station and the strippingroll is increased with a consequent greater risk of premature articleseparation if sheet penetration has been excessive.

Each time any of these problems occurs, the system, of course, must beshut down for clearing with a corresponding resulting loss in output.The extent of the stroke of the platen on which the blades are mountedmust be very closely controlled at all times to obtain just the rightamount of penetration for optimum article separating conditions and thisis especially difficult to do when sheet gage is varied on changing fromone product weight to another. Close operator attention is usuallynecessary, and this is directly contrary to the objective of anautomated system.

SUMMARY OF THE INVENTION Now there have been discovered unique apparatusimprovements in a trim in place thermoforming system for overcoming theabove mentioned deficiencies of the prior art.

Accordingly it is a principal object of this invention to provideapparatus improvements for substantially increasing the capacity of atrim in place thermoforming system.

Another object of this invention is to provide apparatus improvementsapplicable to the trimming phase of such a system.

A further object of this invention is to provide apparatus improvementsfor reducing the criticality of the extent of sheet penetration by thesevering element as it relates to holding of the articles in the sheetafter formation and prior to separation from the sheet.

A particular object of this invention is to provide apparatusimprovements in the cutting or severing ele ment of a trim in placethermoforming assembly.

Other objects will in part be obvious and will in part appearhereinafter.

These and other objects are accomplished in a trim in place,thermoforming process for repeatedly forming articles from heatedportions of a thermoplastic sheet by sealing each heated sheet portionaround a mold, forcing each heated sheet portion within the seal againstthe mold to form the article, penetrating the sheet with a creasingsurface around the periphery of each formed article along the line ofprior sealing to form a line of penetration therein and advancing eachsheet portion with the article attached therein across a strippingsurface to separate each article from each sheet portion along the lineof penetration, by providing the improvement which comprises introducinglimited portions of the creasing'surface into the sheet to a lesserextent than that of the remainder of the creasing surface duringformation of said line of penetration to form frangible tabs on eachside of the portion of the periphery of each article extending in thedirection of advancing movement of the sheet portion, said tabs, becauseof said limited introduction, having a thickness greater than that ofthe thermoplastic along the remainder of the line of penetration,thereby reducing the criticality of sheet thickness along the line ofpenetrationfor retaining the articles attached to the sheet afterforming and prior to article separation.

The means for sealing and penetrating the sheet includes a blade havinga leading surface with formed depressions in portions of its peripherywhich are parallel to the mold sides extending in the direction of sheetadvancement.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENT Major, known components of a trim in place thermoformingsystem are shown and described in detail in U.S. Pat. No. RE 26,4l3, thedisclosure of which is incorporated herein by reference. Thesecomponents include a plurality of molds, each of which has a cavityconforming at least in part to the shape of the article being formed,means, such as pneumatic differential pressure, for forcing heated sheetportions from which articles are to be formed against the cavitysurfaces, a cutting or sealing element, such as a blade, surroundingeach mold for penetrating the sheet around the periphery of eacharticle, and means, such as a piston driven endless chain, for advancingthe sheet with the partially separated articles attached therein acrossa stripping surface such as a breaking roll 44 in FIG. 4 to release eacharticle from the sheet.

Referring now to FIG. 1 of the present drawing, further portions of theapparatus are illustrated. Sealing and trimming are done in the formingpress by moving a platen (not shown) on which the plurality of molds aremounted, toward a rigid, opposed, supporting surface 12 with sheet Sinterposed therebetween. A separate blade 14 is fixedly mounted to theupper supporting platen around the periphery of each mold 10. Each bladehas an end portion protruding slightly beyond its mold 10 towardsupporting surface 12. Though the blades 14 are shown above sheet S, itshould I be obvious that their orientation could be reversed. A spacingmember 16 is provided between adjoining blades. Each mold 10 has aforming surface which includes a cavity which conforms, at least inpart, to the shape of the article being molded. Each mold 10, also hasperipheral portions or sides, illus trated as 18, which extend in thedirection of advancement of sheet 8 through the forming press.Considering this advancement with respect to the illustration of FIG. 1,sheet S is meant to be shown asmoving toward the viewer. Each mold 10also has peripheral portions or sides, not shown, which extend across(from left to right in FIG. 1) or perpendicular to the direction ofsheet advancement through the press.

The article being formed in the illustrated embodiment is a relativelyshallow packaging tray which is rectangular in cross section and whichhas a peripherally extending horizontal flange 28 at the upper end ofits sides when seated in an upright position. Obviously, any articleshape could be formed consistent with the sheet drawing limitations ofthe equipment. In situations where the article periphery, and

consequently the article forming mold, is of a different shape, e.g.,circular, the portions of the mold sidesand of the blade which extend inthe direction of sheet advancement through the press are considered tobe those on either side of a planar center line taken through thearticle in the direction of sheet advancement.

Turning now to FIG. 3, the improved blade of the present invention hasparallel inner and outer side portions 20 and 22, with the outer side 20sloping toward inner side 22 adjacent the forward end portion of eachblade 14, terminating short of intersection with inner side 22 so as todefine a flat, sheet contacting leading surface 24. The rearward endportion of each blade 14 is conventionally secured to the supportingplaten (not shown) as previously described. Leading surface 24 hasdepressions 26 formed therein in the portions of its periphery which areparallel to mold sides 18, the latter, as mentioned, extending in thedirection of advancement of sheet S through the forming press. In theembodiment illustrated, one depression is shown for that portion of theblade periphery parallel to one mold side 18. However, it should beunderstood that a plurality of depressions may be formed in each of theperipheral portions parallel to mold side 18, particularly in forminglarge sized articles No depressions, however, are formed in thoseportions of the blade periphery which are parallel to the mold sideportions extending perpendicular to the direction of advancement of thesheet through the press.

In the illustrated embodiment, each depression 26 has a length extendingin the direction of the blade periphery which is on the order of l/l6inch. Each depression 26 is defined in part by angular surface 34 whichis positioned at an acute angle to outer side 22 of each blade 14. Outerside 22, as shown, is the side of blade 14 which abuts each mold 18, asopposed to side 20 which abuts spacing member 16. In the drawing, thisangle 0 is on the order of 30. Each depression 26 is further defined byopposed vertical sides 30 which extend perpendicularly away from leadingsurface 24 and which each have relatively short vertically extendingedges 32 at one end and longer edges 33 due to angular surface 34 at theother end.

In operation, and as described in part in U.S. Pat. No. RE 26,413,articles are repeatedly formed from heated portions of a generallycontinuous thermoplastic sheet being advanced through the forming presswhich includes the just described components. Each. sheet portion isallowed to contact the surface of lower platen 12 which is at anelevated temperature by reason of its having conventional electricalresistance heaters, not shown, embedded therein, so as to bring eachportion of sheet S which is to define an article up to moldingtemperature. The upper platen on which molds 18 are mounted is thencaused to descend toward supporting surface 12 such that leading surface24 of each blade 14 contacts hot moldable sheet S and seals each heatedsheet portion around its corresponding mold between surface 24 andmember 12. Thereafter, each portion of sheet S within the seal is forcedinto contact with the article defining surface of each mold so as toform the article. Immediately thereafter the platen is again caused todescend for a second time such that blades 14 advance into sheet Saround the periphery of each formed article along the exact line ofprior sealing, in order to form line 36 of penetration therein whichextends through the full thickness of the sheet in the areas other thanwhere indentations 26 are located. However, limited portions of leadingor creasing surface 24 corresponding to the surfaces definingindentations 26 are introduced into sheet S to a lesser extent than thatof the remainder of surface 24 during formation of penetration line 36so as to form angularly oriented frangible tabs 38 in the sheet adjacentthe extremity of each side of the formed tray which extends in thedirection of advancing movement of sheet S through the forming press.Thus, the amount by which each tab 38 is attached to the adjoiningarticle corresponds to surface 32 of indentation 26, is represented by40 in FIG. 2 and is on the order of about 10 percent of the total sheetthickness. With the prior art approach wherein tabs were not employed,penetration was set to be substantially greater i.e., on the order of98-99 percent of the sheet thickness and around the full periphery, inorder to compensate for those areas which were or would be out ofcontrol. Accordingly, tabs 38 because of indentations 26 have athickness greater than that of the thermoplastic along the remainder ofthe line of penetration, the latter preferably being zero, the bladehaving passed entirely through the sheet in this area. Since indentation26 is in part formed by angular surface 34 in the illustratedembodiment, each tab 38 accordingly forms an acute angle with respect tothe adjoining surface 42 of the article being formed, as opposed to theadjoining surface of the web portion of the sheet opposite member 16between adjacently formed articles.

After the second stroke of the platen, which is known in the art as thetrimming stroke, has been carried out, the platen is caused toreciprocate upwardly away from support member 12, the sheet advancemember is activated to move each sheet portion with the formed articlesattached therein at tabs 38 out of the forming press and across adownstream stripping surface such as a horizontally extending edge orroll, As the sheet is passing over the roll, for example in a downwarddirection, the formed article because of its inherent strength willproceed in the direction of sheet advancement just upstream of thestripping surface, whereas the portion of the sheet intermediate thearticles will follow the roll surface because of its flexibility.Accordingly each article will shear away from the surrounding portion ofthe sheet at frangible tabs 38.

As an important facet of the present invention, shearing of tabs 38(FIG. 2) because of their angular configuration will be along the shortside thereof or in other words along the edge of the formed article asopposed to along the edge of the web between adjacent articles. Theremainder of the article has been separated from the sheet in theforming press during the trim stroke of the mold platen. In this way thetabs remain attached to the web rather than the article, and

a separate downstream step to remove them from the articles is avoided.The web with the articles removed therefrom then proceeds in aconventional manner through the nip of takeup rolls where it isrecovered for reuse.

The above description and particularly the drawing are set forth forpurposes of illustration only and are not to be taken in a limitedsense.

The indentations in the leading, sheet contacting surface of the bladeof the present invention may vary in cross sectional configuration fromthat illustrated. The surfaces defining the indentations may be roundedat their intersections with each other in order to facilitate machining.Depending on the nature of the article being formed, the angularconfiguration of the indentations and resulting tabs may not benecessary and the indentation could be rectangular, semi-circular etc.when it is unimportant as to whether the tabs remain with the article orthe web after separation of the article from the sheet. However, whenthe article is a packaging container, such as a tray, which will come incontact with human hands and may, for example, be overwrapped, it ishighly desirable that the edge of the article be relatively free ofprojections, since these may cut fingers and prematurely fracture theoverwrap. Accordingly, the angular configuration of the illustratedembodiment is particularly preferred in such an application, since ithas been found that the tabs remain with the web rather than the articleafter passing across the stripping surface since the thickness of thetab at its joint with the web is greater than at its joint with thearticle. This angle should be between 20 to 60 in order to obtainconsistent separation along the tab joint with the article.

The length of the blade indentations in a direction perpendicular to thedepth, must not be excessive, or else the strength of the tabs will betoo great for shear ing at the stripping station. This length must be atleast equal to or greater than the maximum depth, and this length ispreferably maintained at between one to 50 times the maximum depth ofthe indentations.

With respect to the location of the indentations in the periphery of theblade, as mentioned, they should be located along the sides of thearticle or mold which extend in the direction of sheet advancement asopposed to any corners or to the leading and trailing sides of thearticles which extend generally in a direction perpendicular to that ofsheet advancement. This is so since shearing is facilitated when itoccurs along the line of movement of the sheet as opposed to across theline of movement.

Also, when a number of closely spaced adjacent rows of articles arebeing formed in the press which extend in the direction parallel tosheet advancement, the indentations in adjacent blades should preferablybe staggered with respect to each other rather than aligned since in thelatter case wherein all articles would be sheared from the sheet atsubstantially the same time, there is a distinct possibility offracturing the resulting web because of the stress concentration buildupin the web at these points. However, under certain conditions, forexample when adjacent rows are well spaced from each other, this may notoccur.

It is obvious that many variations may be made in the embodiment setforth herein Without departing from the spirit and scope of theinvention as hereinafter claimed.

I claim:

1. In a trim in place, thermoforming apparatus for repeatedly formingpackaging containers from heated portions of a thermoplastic sheet beingadvanced through the apparatus which includes a container mold havingsides extending in the direction of sheet ad vancement, means forforcing each heated sheet portion into said mold to form the containers,means surrounding said mold for sealing each heated sheet portionagainst an opposing rigid surface prior to forming and for penetratingthe sheet around the periphery of each formed container along the lineof prior sealing to form a line of penetration therein and means foradvancing each heated sheet portion with the container attached thereinacross a stripping surface to break each container away from each heatedsheet portion along the line of penetration, the improvement wherein themeans for sealing and penetrating is a blade having surfaces spaced fromeach other defining its thickness, one of the blade surfaces beingsituated adjacent said mold sides, said blade having a leading thicknesssurface with shaped depressions formed therein which increase in depthacross the thickness in a direction from the surface of the bladeadjacent said mold sides to the other thickness defining blade surfacewhereby to form frangible tabs in said sheet abutting the periphery of acontainer during formation of said line of penetration, the thickness ofsaid tabs as a result of the varying depth of the depressions being lessat their joint with the container than at that with the remainder of thesheet.

2. The apparatus of claim 1 wherein the surfaces of the depressionsproviding said decrease in depth form an angle of from 20 to 60 with thesurface of the blade adjacent said mold sides.

1. In a trim in place, thermoforming apparatus for repeatedly formingpackaging containers from heated portions of a thermoplastic sheet beingadvanced through the apparatus which includes a container mold havingsides extending in the direction of sheet advancement, means for forcingeach heated sheet portion into said mold to form the containers, meanssurrounding said mold for sealing each heated sheet portion against anopposing rigid surface prior to forming and for penetrating the sheetaround the periphery of each formed container along the line of priorsealing to form a line of penetration therein and means for advancingeach heated sheet portion with the container attached therein across astripping surface to break each container away from each heated sheetportion along the line of penetration, the improvement wherein the meansfor sealing and penetrating is a blade having surfaces spaced from eachother defining its thickness, one of the blade surfaces being situatedadjacent said mold sides, said blade having a leading thickness surfacewith shaped depressions formed therein which increase in depth acrossthe thickness in a direction from the surface of the blade adjacent saidmold sides to the other thickness defining blade surface whereby to formfrangible tabs in said sheet abutting the periphery of a containerduring formation of said line of penetration, the thickness of said tabsas a result of the varying depth of the depressions being less at theirjoint with the container than at that with the remainder of the sheet.2. The apparatus of claim 1 wherein the surfaces of the depressionsproviding said decrease in depth form an angle of from 20* to 60* withthe surface of the blade adjacent said mold sides.